Year	Number
1971	25.48498
1972	26.14025
1981	20.90263
1982	20.63739
1983	20.86861
1984	20.2989
1985	20.2124
1986	20.95812
1987	21.26363
1988	20.94973
1989	20.01398
1990	17.18294
1991	16.65279
1992	16.21173
1993	16.42035
1994	16.22677
1995	15.86602
1996	15.19959
1999	11.92944
2000	11.38215
2001	10.87339
2002	11.04805
2003	11.10863
2004	11.00715
2005	11.09815
2006	10.9525
2007	10.80633
2008	10.80427
2009	10.54833
2010	10.80488
2011	11.05291
2012	11.35027
2013	12.01328
2014	11.90957
2015	11.98294
2016	11.60931
2017	11.56883